The Earth beneath our feet

Now that we've charted its origins, let's take a look at planet Earth in a little more detail. The centre of the Earth is a chunk of hard metal, mostly iron, called the inner core. It has a radius of about 1200km; as is often pointed out, that is about the size of the Moon. Above that, you find more iron, but this time molten. This outer core is about 2200km thick. It is heated by warmth mainly left over from the gravitational energy emitted as the planetesimals fell together to form the Earth. The movement of this molten metal is the generator that creates the Earth's magnetic field.

Move a little further out and you start to arrive at material looking more like “rock” as we know it. This is called the mantle and is about 2900km thick.

Although the mantle consists mainly of material that we would recognize as rock, it is so hot and under so much pressure that it too is liquid, or at least viscous. Its flow provides the motive power for the movement of the continents, as the upper mantle takes the topmost part of the solid Earth, the crust, with it as it moves. The crust is on average 35km thick.

Finally, the solid Earth is overlain with yet more fluids. The atmosphere and the oceans and seas (the hydrosphere, for Greek enthusiasts) are the most obvious, but don't forget the cryosphere (the frozen bits, ice and snow) and the magnetosphere, the Earth's magnetic field and the charged atomic particles that it captures, mainly from the Sun. And of course there is the Earth's gravitational field, which continues with ever-diminishing force across the whole of the universe.